(PhysOrg) – Each year as temperatures rise across India, farmers look to the sky and pray for rain.

The all-important monsoon forecast becomes a national priority, with more than 70 percent of India's 1.25 billion citizens engaged in agriculture and relying on weather predictions to decide when they will sow their seeds and harvest their crops.

But getting the forecast right remains a challenge, thanks to the complex—and still poorly understood—ways in which South Asia's monsoon rains are influenced by everything from atmospheric and ocean temperatures to air quality and global climate trends. Even the amount of ice in Antarctica is suspected to have an impact.

And it's only getting harder to figure out, scientists say, as the monsoon becomes increasingly erratic.

A new study released Friday in the journal Science Advances helps clear up a bit of the mystery, by showing that man-made climate change is responsible for most of the change seen in ocean surface temperatures near the equator across Asia, which in turn affect regional rainfall patterns, including the Indian monsoon.

By showing that link, the study indicates future ocean warming in the region, which could in turn increase the amount of rainfall during monsoons, strengthen cyclones, and increase precipitation over East Asia.

"This has important implications for understanding changes in rainfall patterns for a large, and vulnerable population across Asia," said oceanographer Evan Weller, who led the research team while he was at Pohang University of Science and Technology in South Korea, before recently shifting to Monash University in Australia. [more]

ABSTRACT: The Indo-Pacific warm pool (IPWP) has warmed and grown substantially during the past century. The IPWP is Earth’s largest region of warm sea surface temperatures (SSTs), has the highest rainfall, and is fundamental to global atmospheric circulation and hydrological cycle. The region has also experienced the world’s highest rates of sea-level rise in recent decades, indicating large increases in ocean heat content and leading to substantial impacts on small island states in the region. Previous studies have considered mechanisms for the basin-scale ocean warming, but not the causes of the observed IPWP expansion, where expansion in the Indian Ocean has far exceeded that in the Pacific Ocean. We identify human and natural contributions to the observed IPWP changes since the 1950s by comparing observations with climate model simulations using an optimal fingerprinting technique. Greenhouse gas forcing is found to be the dominant cause of the observed increases in IPWP intensity and size, whereas natural fluctuations associated with the Pacific Decadal Oscillation have played a smaller yet significant role. Further, we show that the shape and impact of human-induced IPWP growth could be asymmetric between the Indian and Pacific basins, the causes of which remain uncertain. Human-induced changes in the IPWP have important implications for understanding and projecting related changes in monsoonal rainfall, and frequency or intensity of tropical storms, which have profound socioeconomic consequences.